1. Field of the Invention
The present invention relates to piezoelectric ceramic compositions, piezoelectric ceramic elements, and a method for manufacturing the piezoelectric ceramic compositions. The present invention particularly relates to a piezoelectric ceramic composition adapted to a material for piezoelectric ceramic elements such as piezoelectric ceramic filters piezoelectric ceramic oscillators, and piezoelectric ceramic resonators, and to a piezoelectric ceramic device using the same.
2. Description of the Related Art
Piezoelectric ceramic compositions containing lead zirconate titanate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) as a main component are widely used for piezoelectric ceramic elements such as piezoelectric ceramic filters, piezoelectric ceramic oscillators and piezoelectric ceramic resonators. Since piezoelectric ceramic compositions containing lead zirconate titanate or lead titanate as a main component contain a large amount of lead, there is a problem in that the uniformity of products deteriorates because of the vaporization of lead oxide in the manufacturing process. In order to prevent this above problem, a piezoelectric ceramic composition containing no lead or a small amount of lead is used.
Since other piezoelectric ceramic compositions containing a series of compounds represented by formula (Na1xe2x88x92xLix)NbO3 as a main component do not contain lead oxide, the above-mentioned problem does not arise. However, the (Na1xe2x88x92xLix)NbO3 piezoelectric ceramic compositions provide products having a small mechanical quality factor Qm. Thus, there is a problem in that it is difficult to use such piezoelectric ceramic compositions in, for example, piezoelectric ceramic filters requiring a high mechanical quality factor Qm.
Since morphotropic transformation from a crystalline phase stable at room temperature to another crystalline phase stable at a temperature higher than a room temperature occurs at a relatively low temperature, there is a problem in that the resonance frequency changes significantly at the morphotropic transformation temperature in the (Na1xe2x88x92xLix)NbO3 piezoelectric ceramic compositions.
Accordingly, it is an object of the present invention to provide a piezoelectric ceramic composition containing compounds represented by formula (Na1xe2x88x92xLix)NbO3 and having a perovskite structure as a main component, wherein the piezoelectric ceramic composition provides a product having a higher mechanical quality factor Qm.
It is another object of the present invention to provide a piezoelectric ceramic element using the piezoelectric ceramic composition containing compounds represented by formula (Na1xe2x88x92xLix)NbO3 and having a perovskite structure as a main component, wherein the piezoelectric ceramic element has an increased mechanical quality factor Qm.
It is another object of the present invention to provide a method for manufacturing a piezoelectric ceramic composition containing a compound represented by formula (Na1xe2x88x92xLix)NbO3 and having a perovskite structure as a main component, wherein the piezoelectric ceramic composition provides a product having an increased mechanical quality factor Qm.
The present invention provides a piezoelectric ceramic composition including a compound mainly containing Na, Li, Nb and O and having a perovskite structure, as a main component, wherein the main component has a crystal phase in a semi-stable state at room temperature, the crystal phase originally not being stable at room temperature but being stable at a higher temperature.
In the piezoelectric ceramic composition according to the present invention, the crystal phase, which is stable at a higher temperature than room temperature, has a monoclinic form.
In the piezoelectric ceramic composition according to the present invention, the compound is preferably represented by formula (Na1xe2x88x92xLix)NbO3 wherein 0.02xe2x89xa6xxe2x89xa60.3, and more preferably 0.08xe2x89xa6xxe2x89xa60.18.
In the piezoelectric ceramic composition according to the present invention, the compound may be of formula
(1xe2x88x92n)[(Na1xe2x88x92xLix)1xe2x88x92yKy](Nb1xe2x88x92zTaz)O3. nM1M2O3
wherein 0.02xe2x89xa6xxe2x89xa60.3, 0xe2x89xa6yxe2x89xa60.2, 0xe2x89xa6zxe2x89xa60.2, 0xe2x89xa6nxe2x89xa60.1, M1 represents a bivalent metal element and M2 represents a tetravalent metal element. The bivalent metal element can be at least one selected from the group consisting of Mg, Ca, Sr and Ba, and the tetravalent metal element can be at least one selected from the group consisting of Ti, Zr, Sn, and Hf.
The present invention provides a piezoelectric ceramic device having electrodes used for the piezoelectric ceramic and a piezoelectric ceramic including the piezoelectric ceramic composition.
The present invention provides a method for manufacturing the piezoelectric ceramic composition having the steps of polarizing an untreated piezoelectric ceramic composition, and heating the polarized piezoelectric ceramic composition at a temperature equal to or higher than the temperature at which the crystal phase is in a stable state but lower than a temperature at which the compound does not have ferroelectricity.
In the method for manufacturing a piezoelectric ceramic composition, the temperature to which the polarized piezoelectric ceramic composition is heated is preferably within the range of about 250-400xc2x0 C.
The present invention provides a piezoelectric ceramic composition containing a compound containing Na, Li, Nb, and O and having a perovskite structure, as a main component, wherein the compound is represented by formula (Na1xe2x88x92xLix)NbO3. The piezoelectric ceramic composition contains no lead sand has an increased mechanical quality factor Qm. Thus, the piezoelectric ceramic composition is adapted to piezoelectric ceramic elements such as piezoelectric ceramic filters, piezoelectric ceramic oscillators and piezoelectric ceramic resonators. The present invention further provides a piezoelectric ceramic device including the piezoelectric ceramic composition.
The piezoelectric ceramic composition according to the present invention has a crystal phase in a semi-stable state at room temperature and which is stable at temperatures higher than room temperature. Accordingly, morphotropic transformation in the piezoelectric ceramic composition does not occur at the morphotropic transformation temperature and, thus, the resonance frequency does not change significantly.